This invention relates to a data transmission system and more particularly to a transmission system for transmitting an analog signal whose change of slope is provided as a binary signal to a transmitter for transmission over a transmission medium to a receiver where the binary signal is reconstituted as a replica of the original analog signal.
This invention is particularly suited to the seismic recording technique utilized in geophysical exploration. Prior art techniques either digitize a voltage detected by a geophone which is sent to a recording truck over a transmission line or the analog data generated by the hydrophone is transmitted to the recording truck where it is digitized. In both these prior art techniques, it is customary to use a separate transmission cable between each hydrophone and the recording truck. Since recordings are generally made from a large number of geophones spaced from each other by large distances, laying of the cables between the recording truck and the geophones is burdensome, time consuming, and hence expensive.
Current seismic recording systems acquire full precision amplitude data from many listening (geophone) stations at high sample rates. This volume of data is then transferred onto magnetic media for post-processing. The high cost and great complexity of this type of equipment is well known in the seismic acquisition business.
Several years ago, a new recording method was developed which required only the sign bit of the geophone station to be recorded. This method was proved to be geophysically valid, and in some geographic areas it was shown to produce superior results. The sign bit recording allowed the complexity of the recording system to be drastically reduced since only a single bit was required per geophone station instead of the usual 16 to 22 bits for full amplitude precision. This prior art sign bit recording was essentially hard limiting of the analog signal so that the polarity of the bit was determined by the polarity of the analog signal.
The method of the invention also uses a single bit from the geophone station. However, in contrast to the sign bit recording, the method of the invention produces a bit whose sign is determined by the sign of the slope of the analog signal rather than whether it is a positive or a negative amplitude. The method of the invention provides additional information describing the input signal waveform. In common with the prior art sign bit recording, the cost and complexity of the acquisition system is greatly reduced from that of other prior art systems. In particular, the method of the invention reduces the volume of data transferred from the listening point to the recording system. As a direct biproduct, the complexity as well as the cost of the acquisition equipment is greatly reduced.
With a simple listening station as in this invention, the electronics can be allowed to run continuously. This removes the complex control normally required for commands and control and allows sampling rates to be determined by the acquisition system. The method of the invention is suitable for signals in the low frequency range (1 to 500 Hz) which is the frequency range typical of seismic recording signals. The method also assumes a sinusoidal waveform superimposed upon lower frequency sinusoidal waveforms. The invention is capable of high density (+1000 channels), and high sample rates (10,000 Hz). The invention would make possible environmental studies requiring high density, high frequency, shallow surveys. The invention is also suitable for the excavation market for quick sub-surface measurements.